The present invention relates to an imaging lens for forming an image on an imaging element such as a CCD sensor and a CMOS sensor. In particular, the present invention relates to an imaging lens suitable for being mounted on a vehicle onboard camera, a security camera, a video conference camera, a cellular phone, a digital still camera, a portable information terminal, and a network camera and the like.
In these years, it has become rapidly popular to equip a vehicle with a vehicle onboard camera such as a back camera and a drive recorder. The back camera is used for looking at a backside of the vehicle when the vehicle is pulled back, and the drive recorder is used for recording a view in front of the vehicle for a certain period of time to deal with a potential car accident and so on. For the vehicle onboard camera described above, it is highly demanded to enable to watch and see an image in a range being as wide as possible. Therefore, for the vehicle onboard camera, it is necessary to widen an angle of view thereof.
Japanese Patent Reference has disclosed a conventional imaging lens being relatively small as well as having a wide angle of view. The conventional imaging lens is configured with four lenses. In Patent Reference, the conventional imaging lens includes, in order from an object side to an image plane side, a first lens having negative refractive power; a second lens having positive refractive power and a convex surface facing the object side; an aperture; a third lens formed as a positive meniscus lens having a convex surface facing the image plane side; and a fourth lens formed as a positive meniscus lens having a convex surface facing the object side. With the configuration described above, the second lens, third lens and the fourth lens are restricted to have the refractive powers of 1.7 or more, respectively. Accordingly, the imaging lens is able to be downsized and obtain the wide angle of view, being configured with as few as the four lenses.    Patent Reference: Japanese Patent Publication No. 2007-322656
As the resolution of the imaging element is improved drastically, it becomes necessary for the imaging lens mounted on the vehicle onboard camera and the security camera to obtain sufficient optical performance corresponding to the resolution of the imaging element thus improved. According to the conventional imaging lens in Patent Reference, it is possible to attain the wider angle of view with the small number of the lenses. However, when the first lens of the imaging lens has the negative refractive power as described in Patent Reference, field curvature and chromatic aberration caused by the first lens are necessary to be corrected by the other following lenses having the positive refractive power. In the conventional imaging lens, the field curvature and the chromatic aberration are corrected by the three lenses having the positive refractive power. In this case, both of axial and off-axis chromatic aberrations tend to remain in any manner. Therefore, it is difficult to obtain preferred aberration.
It is not a challenge only for the imaging lenses to be mounted on the vehicle onboard camera and the security camera to obtain the wide angle of view and satisfying correction of the aberration simultaneously, but is also a challenge shared with the imaging lenses to be mounted on, for example, the cameras such as the video conference camera, the cellular phone, the digital still camera, the portable information terminal, and the network camera and the like. The above mentioned cameras are required to have the wide angle of view as well as being relatively small.
In view of the problems of the conventional techniques described above, an object of the present invention is to provide an imaging lens that can satisfactorily correct aberrations while having the wide angle of view.
Further objects and advantages of the invention will be apparent from the following description of the invention.